The present invention relates to a control rod which is used for the reactivity control and the power distribution control in a nuclear reactor.
A nuclear reactor has an excess reactivity in anticipation of the lowering of its reactivity due to the burn-up of fuel, etc. In order to operate the nuclear reactor having the excess reactivity under a sufficiently controlled state, a minus reactivity which is necessary for canceling the excess reactivity and further for reliably shutting down the reactor needs to be prepared. The minus reactivity is achieved by inserting a neutron absorber, for example, a control rod into the reactor core. The control rod is also used for absorbing neutrons in a place of high power within the reactor, thereby to flatten the power distribution and to secure a thermal margin during the operation of the nuclear reactor.
A control rod presently employed in a boiling water reactor is a cruciform control rod constructed of control rod sheaths which are made of stainless steel, and neutron absorbing rods in which the powder of boron carbide (B.sub.4 C) is packed in slender tubes of stainless steel. The neutron absorbing rods numbering about 20 are arranged in a row in each of control wings.
The isotope .sup.10 B of boron contained in boron carbide has as large an absorption cross section as about 3800 barns for thermal neutrons, and absorbs neutrons through the (n, .alpha.) reaction. Therefore, the control rod employing the boron carbide has a high reactivity. As other merits, since the boron carbide is comparatively inexpensive and light, the control rod is low in price and light in weight.
In the control rod of the boiling water reactor, neutron radiation amounts within the control rod are not always uniform, but particularly the neutron radiation amounts in the top end part of the control rod and the outer end parts of the wings are large. For this reason, .sup.10 B in these parts degrades much faster than in the other parts.
In the boiling water reactor, the lifetime of the control rod is stipulated to be the point of time at which the reactivity degradation of a position of 1/4 of the effective length of the control rod has reached 10% of the initial reactivity, and the control rods having reached the lifetime or those anticipated to reach the lifetime in the next operation cycle are exchanged at the routine inspection of the nuclear reactor. Usually, the lifetime of the control rod is determined by the point of time at which the top end part of the control rod has reached the aforementioned condition, because the decrease of .sup.10 B at the top end of the control rod is fast as stated before.
Further, in recent years, the prolongation of the operation cycle and the continuous use of the same control rods (an operation in which the control rod pattern is changed less frequently) have been required for achieving enhancement in the plant factor and simplification in the operation. In a case where the control rods presently used are applied to such operation, the number of the control rods to be exchanged every operation cycle becomes large. Control rods of long lifetime have therefore been requested.
As a control rod whose lifetime has been prolonged, Japanese Laid-open Patent Application No. 53-74697 discloses a control rod in which the neutron absorber at the top end thereof is replaced with a neutron absorber of slow degradation such a hafnium and europium. As compared with the boron carbide, however, the hafnium and the europium are as heavy as about 7 times and about 4 times in the specific gravity, respectively, and they are very expensive. Another disadvantage is that they are lower than .sup.10 B in the neutron absorptivity.
In general, it is improper to employ a metallic absorber, e.g., the hafnium as a structural material such as the control rod sheath or the clad tube of the absorbing rod. The structural material has such necessary conditions that its mechanical strength is sufficient because it bears a strength needed for the structure, and that a required shape is readily obtained by plastic working, welding, etc. For example, the hafnium is hard to be worked and is difficult of welding with stainless steel which is the supporting material of the control rod.